This invention relates in general to controlling audio anomalies and in particular to controlling audio anomalies in signals which have an auxiliary audio signal or dual language audio signal adjacent to a main audio signal.
Audio anomalies occur in a variety of signals such as in the television signals of a television system which has an auxiliary audio signal adjacent to a main audio signal. An understanding of the system in which a signal occurs can help define the causes of the audio anomalies. A television system is used to illustrate the audio anomalies which occur with a signal having an auxiliary audio signal adjacent to a main audio signal.
Television system 10 as illustrated in FIG. 1, includes a headend 14 which receives input programming from input sources 12(a-c). Headend 14 combines the programming from the various input sources 12(a-c) into a television signal with multiple channels and modulates and distributes the television signal to subscriber locations (e.g., subscriber location 17) via distribution system 16. The audio options associated with the multiple channels will vary. Some channels will be available in either monophonic or stereo sound, others will only be available in monophonic sound. Some channels may have an optional second audio language. At subscriber location 17 an intermediate device, such as a home communication terminal (HCT) 18 or a set-top, interfaces to distribution system 16 and to receiver 19. Receiver 19 can be a monophonic television, a stereo television, an audio only stereo receiver, or a computer. HCT 18 allows access to the auxiliary audio options as an additional service of television system 10. The options are unique to each television system 10. For example, a television system 10 may have auxiliary audio signaling on all channels, such as a second audio language for every channel. Another television system 10 may have no auxiliary audio signaling, while another may have auxiliary audio signaling on one or more channels. Television signal 20 contains multiple channels. FIG. 2 is a graphic representation illustrating the components of a single channel within multi-channel television signal 20 having auxiliary audio signal 21 adjacent to main audio signal 22. The components also include video signal 24 and color signal 26. The auxiliary audio signal 21 is provided because additional languages and stereo sound are available and in demand by subscribers paying for access to television signal 20. The auxiliary audio signal 21 may be, for example, Near Instantaneously Companded Audio Multiplex (NICAM) audio signals or dual language audio signals. It is important to note that, even in systems designed for multiple audio signals, auxiliary audio signal 21 may not be present on every programming channel in television signal 20. A set-top, HCT 18, or any television can process main audio signal 22. The reception or processing of auxiliary audio signal 21 can be limited by several factors such as the type of receiver 19, system controlled conditional access within television system 10, and the availability of additional audio on auxiliary audio signal 21.
The multiple channels occur at a set spacing based on the format of television signal 20. In Phase Alteration (by) Line type B (PAL-B) and PAL-G formats the channel spacing is 7 MHz. The location of television signal 20 components is also fixed within the total channel allocation based on the format. The location of main audio signal 22 is offset from the video signal by a fixed frequency. For example, the National Television System Committee (NTSC) format uses a frequency offset of 4.5 Megahertz (MHz), a PAL-I format uses a 6.0 MHz frequency offset, and PAL-B and PAL-G formats use a 5.5 MHz frequency offset.
Audio anomalies are most prevalent in PAL-B, -PAL-G, and PAL-B/G hybrid formats due to the close channel spacing, but have occurred in other formats such as PAL-I. A PAL-B signal format is used for all examples unless stated otherwise. In a PAL-B or PAL-G system additional limitations such as a total channel allocation of only 7 MHz impacts the audio signals. The main audio signal 22 is frequency offset from the video carrier by 5.5 MHz. The auxiliary audio signal 21 is frequency offset from the video carrier by 5.85 MHz. With limited space within the total channel allocation, the auxiliary audio signal 21 is located at only 350 Kilohertz (KHz) above the main audio signal 22. Conversely, in a PAL-I system the auxiliary audio signal 21 is located at 552 KHz above the main audio signal 22. In theory the close spacing of the two audio signals is not a problem as the main audio signal 22 would occur right at 5.5 MHz, however when the main audio signal 22 is modulated there is a spread around 5.5 MHz as shown by the shaded region in FIG. 2. The close proximity of the two audio signals in a PAL-B or PAL-G system creates audio anomalies when filtering out one audio signal to receive the other.
One audio anomaly created in the filtering process is a constant tone generated on a television using the main audio, a monophonic audio. The constant tone is referred to as xe2x80x9caudio whistlexe2x80x9d. Audio whistle occurs when a portion of the main audio signal 22 is processed through the auxiliary audio signal processing circuitry or when signal interference occurs due to the layout of the components within HCT 38. Due to the close proximity of the two signals, filters designed to isolate the auxiliary audio signal 21 capture a small portion of the spread main audio signal 22, for example a portion of the main audio signal 22 at 5.5 MHz. Thus, when the signals are recombined in the output of HCT 18, there are two components of the 5.5 MHz main audio signal 22 with different amplitudes and only slightly different or the same frequencies. One of the components is generated by processing main audio signal 22. The other component is the portion of the main audio signal 22 that was processed through the auxiliary audio signal 21 circuitry. The additional component results in the generation of a constant tone signal, the audio whistle. Attempts to eliminate audio whistle have resulted in the creation of additional problems.
One proposed solution involves locking the main audio signal 22 output frequency to the main audio signal 22 input 5.5 frequency. This solution was unsuccessful as the main audio signal 22 input when modulated had signal spread and the resultant signal lock constantly shifted resulting in a loss of volume control.
A second proposed solution involved designing a deeper notch separating the two signals. Created using devices such as sound traps or filters, a deeper notch or gap between the main audio signal 22 and the auxiliary audio signal 21 allows easier separation of the two signals. Easier separation allows for an easier removal of the main audio signal 22 in the auxiliary audio signal 21 processing circuitry, thus no additional main audio signal 22 component remains in the recombined signal. A deeper notch while currently used in the art, creates additional problems such as audio pop. Audio pop is an instantaneous crackle or popping sound on the stereo or dual language audio.
Attempts to solve these audio anomaly problems included expensive sophisticated filters which still often result in audio popping. Modifying the response of the auxiliary audio signal 21 circuitry to create a deeper notch is also currently being done in the art, but has been found to still result in both some level of audio whistle and some audio pop. Indeed, because the causes and solutions of the audio whistle and audio pop are not easily understood, many cable television systems having multiple audio signals suffer from these audio anomalies.
Thus, what is needed is a better understanding of what is causing the audio anomaly problems and a method, apparatus, and system for eliminating them in signals which may have an auxiliary audio signal 21 adjacent to a main audio signal 22. The solution should be easy to implement, cost effective, and not introduce additional problems into HCT 18.